1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode liquid crystal display (LCD) device, and more particularly, to an IPS-LCD device capable of improving a response speed by reducing a falling time of a liquid crystal molecule.
2. Discussion of the Related Art
Various portable electronic devices, such as mobile phones, personal digital assistant (PDA), and note book computers have been developed because of their small size, light weight, and power-efficient operations. Accordingly, flat panel display devices, such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and vacuum fluorescent displays (VFDs), have also been developed. Of these flat panel display devices, LCDs are currently mass produced because of their simple driving scheme and superior image quality.
The LCD device is a display device for displaying a desired image by independently supplying a data signal to pixels arranged in a matrix form according to image information and thus controlling an optical transmittance of each of the pixels. The LCD device is mainly driven by an active matrix (AM) method. The active matrix method serves to drive a liquid crystal molecule by applying a voltage to the liquid crystal molecule through a switching device such as a thin film transistor (TFT) provided at each pixel.
The LCD device can be classified into various types based upon a method in which the liquid crystal molecules are driven. An LCD device of a twisted nematic (TN) mode has been mainly used among the various LCD devices.
The TN-mode LCD device drives liquid crystal molecules in a direction perpendicular to a substrate by turning on/off an electric field so that a director of the liquid crystal molecules can have an angle of 0°˜90° in relation to the substrate. Accordingly, a black/white mode is easily displayed, a fast response is implemented, and a low driving voltage is required.
However, since the TN-mode LCD device drives liquid crystal molecules in a direction perpendicular to a substrate, a narrow viewing angle is obtained. As a result, a color or a brightness of an image is varied according to an arrangement direction or an arrangement angle of the LCD device. In order to solve the problem, an in-plane switching (IPS) mode LCD device for implementing a wide viewing angle, that is, an in-plane electric field applied to a substrate in a horizontal direction, and for driving liquid crystal molecules under a state that a director of the liquid crystal molecules is horizontal to a substrate along the electric field direction has been proposed.
FIG. 1 is a plan view showing a unit pixel of an IPS-LCD device in accordance with the related art. As shown, a gate line 3 and a data line 1 are arranged on a first substrate of an IPS-LCD device in horizontal and vertical directions, respectively, thereby defining a unit pixel. A switching device (T) is formed at a crossing of the data line 1 and the gate line 3. Also, a common electrode 13 and a pixel electrode 15 are formed in the pixel region in parallel with the data line 1.
The switching device T comprises a gate electrode 4 connected to the gate line 3; a gate insulating layer (not shown) deposited on the gate electrode 4 and formed of SiNx or SiOx; a semiconductor layer A formed on the gate insulating layer; an ohmic-contact layer (not shown) formed on the semiconductor layer A; and a source electrode 5 and a drain electrode 11 formed on the ohmic-contact layer, and connected to the data line 1 and the pixel electrode 15, respectively.
The common electrode 13 is connected to a common line 17 for transmitting a common signal, and the pixel electrode 15 is connected to the drain electrode 11 of the thin film transistor T.
A passivation layer (not shown) formed of an inorganic insulating material or an organic insulating material is deposited on the entire substrate including the pixel electrode 15 and the gate insulating layer, and a first alignment layer (not shown) is deposited on the passivation layer. An initial alignment direction of a liquid crystal molecule is determined according to a rubbing direction of the first alignment layer. When a voltage is not applied to the common electrode 13 and the pixel electrode 15, the liquid crystal molecule is aligned along the rubbing direction of the first alignment layer.
Although not shown, a shielding layer for preventing an optical leakage, a color filter for implementing R, G, and B colors, and an overcoat layer are sequentially deposited on a second substrate corresponding to the first substrate. Also, a second alignment layer having the same rubbing direction as the first alignment layer is deposited on the overcoat layer.
A liquid crystal layer for controlling an optical transmittance by a voltage applied to the common electrode 13 and the pixel electrode 15 is formed between the first substrate and the second substrate.
FIGS. 2A and 2B show an operation of the IPS-LCD device, in which FIG. 2A shows a driving state of a liquid crystal molecule 50 when a voltage is not applied between the common electrode 13 and the pixel electrode 15, and FIG. 2B shows a driving state of the liquid crystal molecule 50 when a voltage is applied therebetween.
As shown in FIG. 2A, when a voltage is not applied between the two electrodes 13 and 15 of the IPS-LCD device, the liquid crystal molecule 50 inside the liquid crystal layer is aligned along the rubbing direction of the first and second alignment layers deposited on facing surfaces of the first and second substrates (the arrow direction of the dotted line in drawing). As the result, a normally black mode is displayed.
As shown in FIG. 2B, when a voltage is applied between the two electrodes 13 and 15 of the IPS-LCD device, a horizontal electric field is generated therebetween. The liquid crystal molecule 50a is twisted by a maximum angle of 45° thereby to transmit light.
When a voltage is applied to the common electrode 13 and the pixel electrode 15, a horizontal electric field parallel with the substrate is generated between the common electrode 13 and the pixel electrode 15. Accordingly, the liquid crystal molecule is aligned in a parallel state with the substrate along the horizontal electric field, thereby having a viewing angle wider than that of the related art TN LCD device.
However, the IPS-LCD device has a response speed slower than that of the related art TN-LCD device, thereby having a difficulty in processing a moving image. In order to solve the problem, several methods for accelerating a response speed of a liquid crystal molecule have been proposed. One of the methods is to easily displace the liquid crystal molecule against a torque applied from an electric field by lowering a viscosity of the liquid crystal molecule.
That is, in the IPS-LCD device, an elastic torque of the liquid crystal molecule is much lowered than an electric torque of the liquid crystal molecule by degrading a viscosity of the liquid crystal molecule. As the result, each driving speed of the liquid crystal molecule is accelerated by the lowering of the elastic torque when the electric torque is constant.
However, since the elastic torque of the liquid crystal molecule is lowered, a falling time of the liquid crystal molecule is increased in an off state field. That is, the decrease of the viscosity of the liquid crystal molecule causes a decrease of a restoration force of the liquid crystal molecule to return to the initial position from a distorted state, thereby increasing the falling time of the liquid crystal molecule.
In order to improve the response speed of the liquid crystal molecule, both a rising time taken for the liquid crystal molecule to be distorted by an electric field and a falling time taken for the liquid crystal molecule to be restored to the initial position have to be considered. Therefore, the entire response speed of the liquid crystal molecule can not be sufficiently improved only by the lowering of the viscosity of the liquid crystal molecule.